Do things while the opportunity or momentum exists; literally "forge the iron while it's hot," meaning act promptly or train early.
子どものうちに語学を学ばせるべきだ。鉄は熱いうちに鍛えよというからね。
You should have children learn languages early. After all, you should strike while the iron is hot.
新製品の反応が良い今こそ広告を強化するチャンスだ。鉄は熱いうちに鍛えよ。
Now that the new product is getting a good response, it's the chance to strengthen advertising. Strike while the iron is hot.
覚えたてのうちに復習したほうが身につく。鉄は熱いうちに鍛えよだよ。
It's better to review while it's fresh in your mind. You should strike while the iron is hot.